1. Technical Field
This application relates to computer storage devices, and more particularly to the field of determining access rates of different portions of storage devices.
2. Description of Related Art
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units (host adapters), disk drives and other physical storage, and disk interface units (disk adapters). Such storage devices are provided, for example, by EMC Corporation of Hopkinton, Mass., and disclosed in U.S. Pat. No. 5,206,939 to Yanai et al., 5,778,394 to Galtzur et al., U.S. Pat. No. 5,845,147 to Vishlitzky et al., and U.S. Pat. No. 5,857,208 to Ofek. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels of the storage device and the storage device provides data to the host systems also through the channels. The host systems do not address the physical storage of the storage device directly, but rather, access what appears to the host systems as a plurality of logical volumes. The logical volumes may or may not correspond to the actual disk drives and/or other physical storage.
In some cases, it may be useful to provide multiple tiers of physical storage where each tier corresponds to memory storage having a particular speed associated therewith. Note that faster memory is usually more expensive, so it is often more cost effective to combine fast and slow memory. System throughput may be improved by placing data that is more frequently accessed in relatively faster physical storage. However, conventional techniques for monitoring data accesses may not be useful in very large systems. For example, in a system with one terabyte of storage, a table for monitoring megabyte chunks of the memory would have one million entries. On the other hand, increasing the size of the chunks, while decreasing the table size, may make the chunks too big to be able to ascertain useful access frequency information. For example, a gigabyte chunk may have a relatively dormant section and a relatively active section. In such a case, the chunk may be treated as having a medium access rate, even though this is not true for either subsection. In addition, monitoring data on a per file basis may not be useful, since in some cases it is possible to have some portions of files be significantly more active than other portions, especially in the case of relatively large files.
Accordingly, it is desirable to provide a mechanism that can obtain and use information about data access rates in a way that does not require a large table of entries but still addresses granularity issues associated with chunks that are too large.